It has recently been proposed a process for removing heavy metals from petroleum hydrocarbons containing such heavy metals and cracking the petroleum hydrocarbons by contacting them, at a temperature within a specified range, with a substance obtained from nickel ore baked at a temperature between 350.degree. C and 600.degree. C (refer to Japanese Patent Laid Open Publication No. 10,202/1974). The substance for use with the above process prepared by baking the nickel ore at a temperature between 350.degree. C and 600.degree. C has, however, only an insufficient physical strength and easily pulverizes to result a significant catalyst loss due to the pulverization in the process of cracking heavy oils using the above substance as the catalyst. The use of the above substance as the catalyst for cracking the heavy oils is therefore economically disadvantageous.